Piezoelectric devices alter their shape in response to an applied electric field. An electric field applied in the direction of polarization affects an expansion of the piezoelectric material in the same direction, while a voltage applied in the opposite direction of polarization will cause a contraction of the material in that same direction. Piezoelectric devices, such as thermally pre-stressed benders, use the bending action of the bender to convert electrical energy into mechanical energy. In other words, when a voltage is applied across a piezoelectric bender, the bender flattens and produces an axial force. When an opposite voltage is applied, the piezoelectric bender increases its curvature, and produces an axial force in an opposite direction.
Engineers have observed that the magnitude of an axial force from a given piezoelectric bender is related to the stiffness of the bender. Stiffness can be adjusted by restricting the transverse expansion and contraction of the piezoelectric bender, such as by using a peripheral clamp of a type generally described in co-owned U.S. Pat. No. 6,376,969 to Forck. Forck seeks to increase stiffness of a piezoelectric bender by clamping around its peripheral edge. In addition, Forck compensates for temperature changes by including a temperature responsive element in its clamp, so that the piezoelectric bender operates similarly across a range of temperatures. Forck in essence teaches a static clamping load around the periphery of the piezoelectric bender. This static clamping load varies with temperature due to the inclusion of a temperature responsive element in the clamp.
More recently, engineers have observed that, due to geometrical changes occurring at the peripheral edge of the piezoelectric bender, that the clamp may not remain clamped to the piezoelectric bender throughout its deformation. The consequence of this phenomenon is a change in the stiffness of the piezoelectric bender at different points in its deformation, which results in decreased actuating forces from the piezoelectric bender, especially at the extreme deformation. In other words, as the piezoelectric bender flattens due to the application of an electrical voltage, the profile of the piezoelectric bender at the peripheral edge becomes slightly thinner due to that increasing flatness. That slight change in the profile thickness of the piezoelectric bender can be sufficient to cause one side or the other of the piezoelectric bender to lose contact with a clamping surface, resulting in a decreased resistance to the deformation, and hence a reduced actuating force from the piezoelectric bender.
Another potential problem associated with piezoelectric benders relates to wear at the surfaces where the peripheral edge of the piezoelectric bender comes in contact with a clamp surface or other housing surface. Over time, wear at these surfaces can also alter the geometry in the peripheral region, and hence alter the stiffness and performance of the piezoelectric bender. In extreme cases, the wear can become so severe that the piezoelectric bender becomes substantially unclamped over its entire deformation range, resulting in a relatively drastic reduction in the actuating force available. Thus, without some means for compensating for wear, the actuating force produced by a piezoelectric bender could gradually decrease over its operational lifetime, potentially undermining the operation of the device to which the piezoelectric bender is coupled. For instance, if the piezoelectric bender is used in conjunction with a valve, the loading of the valve member on a valve seat could gradually decrease over the lifetime of the piezoelectric bender, eventually resulting in valve seat loading that drops below that necessary to keep the valve seated.
The present invention is directed to overcoming one or more of the problems set forth above.